I have a problem with Grimm. We are currently visiting my mother and father and Grimm an Argon came along. Grimm is skittish withe new people, but was approaching both of my parents and asking for pets. Yesterday, I left him with my father while my mother and I shopped.
Now, this all comes second hand, but apparently my father was sitting down watching television, and Grimm was lying in the doorway about 6 feet away not doing much. Then Dad heard a strange noise. He turned off the TV, looked around and realized that Grimm was growling at him.
Grimm was laying down, he was not snarling, and didn't even seem to be looking at my fathe the entire time, but Dad swears it was growl, not a snore or grunt. Just a low, constant rumbling.
Dad didn't get up or try and intimidate him for 10 minutes or so. Then he asked Argon and Radar (my parents dog) if they would like to go for a walk. They did and when they went by Grimm, he got up also.
Dad walked the other two, but Grimm would not allow Dad to put a leash on him. He was not aggressive at all, but he just kept backing away. Rather then upset him further, Dad just left him alone and called Mom and I to home home and deal with it.
Grimm is fine with my father when I am present. He will approach him appropriately, especially if he thinks Argon is getting attention. It's when I'm not there that the problems start.
I've been having Dad feed Grimm and offer him treats for coming over, but what else can I do? Obviously, this is an issue that needs to be addressed before Grimm can be adopted.
I've known that Grimm is not as social when I am not present, even with coworkers that he knows, but I have only had him a few weeks, and this is the first time he has ever growled.

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Booty Dancing In Heaven 10/13/03-9/15/12As much as I try to be one of those easygoing 'spread your wings and fly' types, I just can't stop trying to burst people into flames with my mind.

My boyfriend has a pitbull who growls a lot, even if he seems happy. I'm not really a reputable source of information, I'm just talking from experience, but I'll keep petting him even if he is growling and reassure him everything is okay (this dog is very familiar and comfortable with me by the way). Maybe you should be around when he's growling at your dad, and both you and your dad pet him reassuring him it's okay, then once he stops growling have your dad give him a treat?

Petting and reassuring a growling dog is rewarding him for growling, guaranteeing the behavior will continue. Dogs almost always growl for a reason. I would try and assertain the reason and then work from there. Growling is not a bad thing. If he's trying to tell you something, and he probably is, you should listen.